(1) Field of the Invention
The invention relates to a time division multiplex telecommunication exchange, comprising a number of line circuits to which incoming and outgoing transmission channels are connected, which transmission channels are arranged for the transmission of bit stream, which are divided into bit groups, and in which a switching device is available for the transmission in time division multiplex of bit groups from the line circuits of the incoming transmission channels to the line circuits of the outgoing transmission channels through a common bus line.
The invention relates in particular to data switching exchanges for the selective exchange of data between connected transmission lines. The data signals may be isochronous binary signals or also asynchronous start-stop telegraph signals.
(2) Description of the State of the Art
Data switching exchanges are known in which all data which are exchanged between the connected transmission lines are transmitted through a central bus line, which is accessible in time division multiplex to all line circuits. An example of such a data switching exchange is described in French patent specification No. 2,119,417.
In the line circuits of the incoming transmission lines the data bits are collected in bit groups. A bit group which must be transferred from an incoming transmission line to an outgoing transmission line passes the central bus line in a time slot which is much smaller than the time slot occupied by the bit group on the transmission line. In this way, many bit groups can be transferred through the central bus line within the duration of a time slot on a transmission line.
The number of time slots on the central bus can be chosen equal to the number of incoming transmission lines, in which case each incoming transmission line may have its own time slot on the bus. In this case no blocking can occur.
When the number of connected transmission lines is great, the duration of the time slots in the bus is very short. The maximum obtainable switching speed of the electronic circuits then puts a natural limit to the number of transmission lines that can be connected. This number can only be increased if a certain possibility of blocking is accepted. In subscriber systems in which the traffic per subscriber is low and where the traffic is of a probability character, it is possible to reduce the number of time slots on the bus with respect to a number of subscriber's lines.
The ratio between the number of connected subscribers' lines and the number of time slots is the concentration factor. In telephone systems for normal subscribers, this factor is of the order of 10 for the first concentration stage. There is then, however, a certain probability that the subscriber cannot get a connection. In view of the probability character of subscriber traffic, big concentration factors can be effected for a given maximum possibility of blocking.
In the case of transmission lines carrying the traffic of a group of subscribers, it depends on the traffic capacity, which can be roughly defined as the percentage of the time the line is occupied by traffic in a busy hour, in how far a still further concentration can be effected.
Time division multiplex data switching exchanges with concentration can be distinguished in a type, in which a particular time slot is associated with each connection, and in a type with so-called addressed time division multiplex, in which any arbitrary time slot is made available for the transfer of each bit group. In the latter case each bit group can be provided with a destination address, which indicates to which outgoing transmission line the bit group must be transferred.